Vapor Separation
Gas streams containing condensable vapors, such as water vapor, sulfur dioxide, ammonia or organic vapors, arise from numerous industrial and commercial processes. One method of removing the vapor from the gas stream is by means of a membrane separation step, followed by condensation of the vapor-enriched stream from the membrane separation step.
A typical membrane vapor separation system includes a membrane unit, a pump for lowering the pressure on the permeate side of the membrane, and a condenser for liquefying the vapor. Membrane processes for removing vapors from gas streams are described, for instance, in U.S. Pat. Nos. 3,903,694, 4,553,983 and 4,906,256, which all deal with removal of organic vapor from air or other gases, U.S. Pat. No. 4,444,571, which deals with removal of water vapor from gas streams, and U.S. Pat. Nos. 4,606,740 and 4,608,060, which describe membranes for removing polar gases such as hydrogen sulfide, sulfur dioxide and ammonia from other gases.
In a vapor removal process characterized by membrane separation followed by condensation, the vapor concentration in the condenser vent gas after the condensation step depends on the vapor/liquid equilibrium at the operating conditions under which the condensation is performed. It is frequently the case that the condenser vent gas contains a much higher concentration of vapor than the original feed gas. The vent gas is often recirculated to the feed side of the membrane unit for further treatment. This type of scheme, performed via an oven, is shown for example, in U.S. Pat. Nos. 4,553,983.
There are several problems associated with returning the condenser vent gas to the membrane feed. First, the more concentrated is the vent gas compared with the feed gas, the less efficient the system becomes. Suppose, for example, the feed gas contains 2% vapor, the vapor-enriched stream from the membrane separation step contains 20% vapor, and the vent gas from the condenser contains 10% vapor. Then about half of the amount of vapor removed and concentrated by the membrane is recirculated to the front of the membrane. Much of the separation achieved by membrane is then negated, resulting in increased membrane area and pump capacity requirements for the system.
What can be done to handle the condenser vent gas stream is to pass it to a second membrane stage. This stage can be designed to produce a discharge stream with a concentration about the same as the original feed, so as to minimize the impact of the recycled stream on the total process. The vapor-enriched stream from the second membrane stage is in turn condensed, and the vent gas from the second condenser is returned to the feed of the second membrane unit. Such an arrangement is shown, for example, in U.S. Pat. No. 4,906,256, FIG. 3. A two-stage system is complex compared with a one-stage, uses more controls and is more costly, since two sets of most components are needed.